In the compressor section of an axial flow turbomachine, there are both vanes and blades. The vanes, also called stators, are static structures and have the function of turning the direction of airflow. The cross section of a vane is in the shape of an airfoil and a typical part may have a length of about 50-80 mm, a chord length of about 25 mm and a maximum cross section thickness of about 2.5 mm. A vane assembly is comprised of a multiplicity of vanes disposed radially around the circumference of an annulus. In a typical vane assembly, there are inner and outer circumferential ring members, both made of sheet metal. The vanes are fitted and brazed into slots in both members, connecting them together radially, much as the spokes of a wheel connect the rim to the hub. In advanced gas turbines, some of the vanes are made of high temperature capability nickel alloys, for use in the hotter high pressure portions of the compressor. Vanes must be manufactured to precise dimensions and must be accurately placed in their assembly for engines to obtain highest efficiency. But nickel alloys, especially superalloys are often difficult to work and it has not been easy to precisely shape the airfoil.
While full machining of a solid could be used, precision forging has been preferred for economic reasons in the production of vanes. However, precision forging is limited in the accuracies obtainable and the tolerance variation must be accommodated in the fits of the circumferential rings into which the vanes are received and brazed. The resultant fit allowance means that the size of the braze gap is greater than desired and there will be more variation in vane alignment than may be optimal. The more there is deviation from the theoretical perfection of the design, the less the efficiency of the engine. Thus, there has been a need to make airfoils in a more precise manner and it is to this that the present invention is directed.
The present invention involves rolling of sheet metal and there is much technology relating to such. But a particular pertinent part of the prior art is reflected in U.S. Pat. No. 4,320,647 to Kummeling et al. The patent describes the forming of door hinges by stamping them from a contour rolled piece of metal strip. The strip rolling process involves making a cross section in which two opposing similar cross sections are divided by a groove running in the longitudinal direction of the strip. The longitudinal groove is used as a guide for locating the strip when it passes through a stamping die. Somewhat similar procedures have been used in part of the present invention. But improvements have been necessary to accurately produce the more precise and more complex cambered structure which constitutes a gas turbine engine vane.